The typical asking rate per listing is in the $0's per square foot per year. The O'Fallon region is going through a decline in rates. In the last 3 months, the median rate fell $20 per sqft per year. That's a 100% decline in rent.
In the last 90 days, 31 listings have been posted online, which means leases most likley are leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around sqft. Most properties in this market are around 1 stories. Most buildings in this area were built in 2001 and are Class A.